Cyanate ester compounds are compounds that form triazine rings by curing. Cyanate ester compounds are widely used as starting materials for various functional polymer materials such as composite materials for structures, adhesives, electrical insulating materials, and electric and electronic components, because of their high heat resistance and excellent electrical properties. In recent years, however, there are increasingly strict requirements concerning properties of functional polymer materials as a higher level of performance is required in these fields of application. Examples of such various properties include flame retardancy, heat resistance, low coeffiencies of thermal expansion, low water-absorbing property, low dielectric constant, low dielectric loss tangent, weather resistance, chemical resistance, and high fracture toughness. However, requirements concerning these properties have not always been satisfied so far.
For example, in the field of semiconductor packaging materials, undesired warpage occurs due to the mismatched coefficiency of thermal expansion between semiconductor chips and substrate materials with thinning of substrates. As an approach for solving this problem, the functional polymer materials themselves for use in substrate materials are required to have lower thermal expansion and higher heat resistance. Furthermore, use of lead-free solder is promoted for the soldering of printed circuit boards, in consideration of human bodies and environments. In response to this, the functional polymer materials themselves are also required to have lower thermal expansion and higher heat resistance because of being capable of resisting a reflow step at a high temperature.
Conventional functional polymer materials may be allowed to contain a halogen atom or a phosphorus atom from the viewpoint of enhancing the flame retardancy of the functional polymer materials. However, the halogen atom has the possibility of generating halogen gases, which might cause environmental pollution, during combustion. In addition, the halogen atom reduces the insulating properties of final products. Also, the phosphorus atom often reduces the required properties (heat resistance, moisture resistance, and low water-absorbing property, etc.) except for flame retardancy. Accordingly, there is also a demand for improving the flame retardancy of the functional polymer materials without containing a halogen atom and a phosphorus atom.
A prepreg may be used when producing a laminate for use in printed circuit boards, etc. A prepreg may be produced by, first, dissolving an uncured monomer, which is a precursor of a functional polymer material, in a solvent such as methyl ethyl ketone to prepare varnish, impregnating glass fiber with the varnish, and drying the varnish. Therefore, there is also a demand for improving the solvent solubility of the monomers.
In the field of semiconductor sealing materials, studies have been actively conducted to replace silicon (Si) semiconductor devices with wide-gap semiconductors such as silicon carbide (SiC) semiconductors with the aim of reduction in power loss (energy saving). The SiC semiconductors are more chemically stable than the Si semiconductors and therefore permit operation at a high temperature exceeding 200° C., and it is also hoped that the apparatus size is reduced. In response to the replacement, compositions comprising functional polymer materials for use in sealing materials are required to have heat resistance, low thermal expansion, and heat resistance at high temperatures over a long period (hereinafter, referred to as “long-term heat resistance”), etc.
In order to obtain a hardened product of a cyanate ester compound alone which possesses low thermal expansion and heat resistance, use of a bifunctional cyanatophenyl-based cyanate ester compound in which a hydrogen atom of a methylene group that bonds cyanatophenyl groups is replaced by a particular alkyl group (1,1-bis(4-cyanatophenyl)isobutane) has been proposed (see Patent Document 1).
In order to obtain a hardened product of a cyanate ester compound alone which possesses flame retardancy and/or heat resistance, the use of a cyanate ester compound containing an isocyanuric acid skeleton (see Patent Document 2), a cyanate ester compound containing a triazine skeleton (see Patent Document 3), a bifunctional cyanatophenyl-based cyanate ester compound in which a hydrogen atom of a methylene group that bonds cyanatophenyl groups is replaced by a biphenyl group (see Patent Document 4), or a cyanation product of a phenol-modified xylene formaldehyde resin (see Patent Document 5); and a combination of a trifunctional cyanatophenyl-based (trisphenolalkane-based) cyanate ester compound and a bifunctional cyanatophenyl-based cyanate ester compound (see Patent Document 6) or a combination of a bisphenol A-based cyanate ester compound and an imide skeleton-containing cyanate ester compound (see Patent Document 7) have been proposed.
A composition containing a novolac-based cyanate, a novolac-based phenol resin, and an inorganic filler has been proposed as a composition which has heat resistance and high-temperature heat resistance over a long period (long-term heat resistance) and in which a cyanate ester compound is used (Patent Document 8).